The present invention is related to hydrogen storage alloy electrodes. In particular, the present invention is related to a method of activating hydrogen storage alloy electrodes.
Hydrogen storage alloy materials are used in a variety of applications. Examples of such applications include both rechargeable electrochemical cells as well as fuel cells. Rechargeable electrochemical cells using a hydrogen storage alloy as the active material for the negative electrode are known in the art. The negative electrode is capable of the reversible electrochemical storage of hydrogen. The positive electrode typically comprises a nickel hydroxide active material although other active materials, such as manganese hydroxide, may be used. The negative and positive electrodes are spaced apart in an alkaline electrolyte. A suitable separator (i.e., a membrane) may also be positioned between the electrodes. As used herein the terminology xe2x80x9cmetal hydride materialxe2x80x9d, xe2x80x9chydrogen storage alloyxe2x80x9d, and xe2x80x9chydrogen absorbing alloyxe2x80x9d are synonymous.
Upon application of an electrical current to the negative electrode, the active metal hydride material is charged by the absorption of hydrogen. This is shown by reaction (1).
M+H2O+exe2x88x92xe2x88x92 greater than M-H+OHxe2x88x92 (Charging)xe2x80x83xe2x80x83(1)
Upon discharge, the stored hydrogen is released by the metal hydride material to provide an electric current. This is shown by reaction (2).
M-H+OHxe2x88x92xe2x80x94 greater than M+H2O+exe2x88x92 (Discharging)xe2x80x83xe2x80x83(2)
The reactions at a conventional nickel hydroxide positive electrode as utilized in a nickel-metal hydride electrochemical cell are as follows:
Ni(OH)2+OHxe2x88x92xe2x88x92 greater than NiOOH+H2O+exe2x88x92 (Charging)xe2x80x83xe2x80x83(3)
NiOOH+H2O+exe2x88x92xe2x88x92 greater than Ni(OH)2+OHxe2x88x92 (Discharging)xe2x80x83xe2x80x83(4)
Based on the pioneering principles of Stanford R. Ovshinsky, a family of extremely efficient electrochemical hydrogen storage materials were formulated. These are the Tixe2x80x94Vxe2x80x94Zrxe2x80x94Ni type active materials such as those disclosed in U.S. Pat. No. 4,551,400 (xe2x80x9cthe ""400 Patentxe2x80x9d) the disclosure of which is incorporated herein by reference. These materials reversibly form hydrides in order to store hydrogen. All the materials used in the ""400 Patent utilize a generic Tixe2x80x94Vxe2x80x94Ni composition, where at least Ti, V, and Ni are present with at least one or more of Cr, Zr, and Al.
Other examples of metal hydride alloys are provided in U.S. Pat. No. 4,728,586 (xe2x80x9cthe ""586 Patentxe2x80x9d) the disclosure of which is incorporated herein by reference. The ""586 Patent describes a specific sub-class of these Tixe2x80x94Vxe2x80x94Nixe2x80x94Zr alloys comprising Ti, V, Zr, Ni, and a fifth component, Cr. The ""586 patent, mentions the possibility of additives and modifiers beyond the Ti, V, Zr, Ni, and Cr components of the alloys, and generally discusses specific additives and modifiers, the amounts and interactions of these modifiers, and the particular benefits that could be expected from them. Still other examples of hydrogen absorbing alloys are provided in U.S. Pat. No. 5,536,591 (xe2x80x9cthe ""591 Patentxe2x80x9d), the disclosure of which is incorporated herein by reference. In particular, the ""591 Patent provides teaching on the type of surface interface at the metal hydride electrode and the nature of catalytic sites ideal for promoting high rate discharge.
In part, due to the research into the negative electrode active materials, the Ovonic nickel-metal hydride batteries have demonstrated excellent performance characteristics such as power, capacity, charging efficiency, rate capability and cycle life. Presently, there is an increasing use of rechargeable nickel-metal hydride batteries in all types of portable tools, appliances, and computer devices. As well, there is a growing use of nickel-metal hydride cells in applications such as electric and hybrid-electric vehicles. Many of the new uses for the nickel-metal hydride cells require that further improvements be made in the cell""s performance.
One of the crucial steps in the preparation of a hydrogen storage alloy electrode is that of xe2x80x9cactivationxe2x80x9d. Activation increases the rate at which the hydrogen storage alloy reacts with hydrogen or the extent to which hydrogen is incorporated into the alloy to form the metal hydride.
Activation is believed to result from 1) removal of reducible surface oxide which tends to interfere with the functioning of the material, 2) reduction of particle size resulting from an increase in volume, which fractures the alloy particles, and 3) changes in the chemical composition and/or structure of the alloy or the surface of the alloy. Activation, it is believed, increases the surface area and alters the chemical composition of the alloy surface.
Activation may be achieved through the surface treatment of the electrode by subjecting the electrode to an alkaline or acidic etching treatment. This type of surface treatment alters the surface oxides of the hydrogen storage alloy. An example of a hot alkaline etch treatment is provided in U.S. Pat. No. 4,716,088, the disclosure of which is incorporated by reference herein. Another form of activation is a pulse-potential process as described in U.S. Pat. No. 5,560,752, the disclosure of which is also incorporated by reference herein. This activation process applies an alternating hydriding/dehydriding potential to the electrode. The present invention is directed to an alternate activation process which uses current pulses to activate the hydrogen storage alloy electrode.
Disclosed herein is a method of activating a hydrogen storage alloy electrode, comprising the step of:
applying a plurality of current cycles to the electrode, each of the current cycles including a first pulse effective to at least partially charge the electrode and a second pulse effective to at least partially discharge the electrode.
Also disclosed herein is a method of activating a fuel cell electrode including a hydrogen storage alloy, the method comprising the step of:
applying a plurality of current cycles to the fuel cell electrode, each of the current cycles including a first pulse effective to at least partially charge the electrode and a second pulse effective to at least partially discharge the electrode